use EVENTDNS_H to check for evdns

--HG--
extra : transplant_source : %FBn%9Bh%A4%04%FD/%8C%5C%C1%DC%A01%9C%F7%A8v%9B%B4

gevent/core.pyx

@@ -375,6 +375,7 @@ def get_method():
 
 cdef extern from *:
     cdef void emit_ifdef "#if defined(_EVENT_VERSION) //" ()
+    cdef void emit_evdns_ifdef "#if defined(EVENTDNS_H) //" ()
     cdef void emit_else  "#else //" ()
     cdef void emit_endif "#endif //" ()